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The Mediterranean ecosystem

The Mediterranean Sea has a relatively high species diversity, largely due to its long evolutionary history and the prehistoric introduction of many Atlantic species into the
Mediterranean. The present-day high species richness is due to spatial coexistence of warm water species (thriving in the summer) and cold-water species (thriving in the winter). This seasonal change in species activity is a buffer against the effects of environmental variation, because a varied set of species is more likely to adjust to environmental change.[1]

Previous changes

Since the 1980s, the Mediterranean marine biota have experienced rapid, dramatic changes, illustrated by alteration of food webs, mass mortalities, and population explosions such as jellyfish outbreaks. These changes are caused by intense anthropogenic activities, but also by climate change.

The advance of warm-water species represented the first evidence of a linkage between climate change and distribution patterns in the Mediterranean Sea. This phenomenon is particularly evident in fish, where over 30 native (warm-water) species have already spread into northern areas. Almost all of the 100 fish species newly recorded in the Mediterranean are of warm-water affinity. At the same time, the physical properties of the
basin have changed and temperatures have increased.[1]

This figure reconstructs the history of ecosystem functioning within the Adriatic Sea in the last 30 years. The initial microbial pathway sustained the crustacean-fish pathway and lead to a very productive fisheries (left). Other pathways linked to global warming, lead to scenarios and where the yield in fisheries is not as high as previously.

Predictions

Endemic native species with cold-water affinity, common in the northern part of the Mediterranean, will probably decline and eventually be lost. A decline in their occurrence
has been reported already. It is also possible that some of these species might become
adapted to the new conditions, after periods of stress.

The seaweed Fucus virsoides, an endemic flagship species of the Northern Adriatic (the coldest portion of the Mediterranean Sea), appeared to suffer severe stress in former years, whereas it is now particularly abundant, for example in Venice.
In general, the recent warming has facilitated the establishment and distribution of tropical, exotic species that have been introduced either via the Suez Canal or by maritime transport. This process is fast advancing, and more than 500 non-indigenous species have already been recorded in the Mediterranean. Some undoubtedly raise some concern, such as the jellyfish Rhopilema nomadica – which even shut down a nuclear power plant by clogging its cooling system – whereas others are becoming a resource for fisheries. Entire replicas of tropical communities from the Red Sea have already been recorded from a few Mediterranean locations.

From this it can be concluded that if the Mediterranean continues to warm at the same rate, all its sub-regional, biological peculiarities may rapidly disappear, to be replaced by a more homogeneous, tropical-like ecosystem.[1]